There is an increasing demand for clean burning high performance fuels. Distillate fuels, e.g. straight-run light naphtha, typically contain paraffins, naphthenes and aromatics. Naphthenes, i.e. cyclic paraffins, such as methylcyclopentane (MCP) and cyclohexane (CH) have low octane numbers (RON) of 91 and 83, respectively. If the rings are opened and isomerized, the resulting isomerized paraffins have higher octane numbers. Paraffins also have a lower density than the corresponding naphthenes. Thus, there is a need for improved catalysts for ring opening.
An increased amount of paraffins is also required in providing reformulated gasoline. Reformulated gasoline differs from the traditional product in having a lower vapor pressure, lower final boiling point, increased content of oxygenates, and lower content of olefins, benzene and aromatics.
Reduction in gasoline benzene content often has been addressed by changing the cut point between light and heavy naphtha and directing more of the potential benzene formers to isomerization instead of to reforming. No benzene is formed in isomerization, wherein benzene is converted to C6 naphthenes and C6 naphthenes are isomerized toward an equilibrium mixture of cyclohexane and methylcyclopentane or converted to paraffins through ring opening. It is believed that such C6 cyclics are preferentially adsorbed on catalyst sites relative to paraffins, and the cyclics thus have a significant effect on catalyst activity for isomerization of paraffins. Refiners thus face the problem of maintaining the performance of light-naphtha isomerization units which process an increased concentration of feedstock cyclics.
Ring opening is the preferred reaction to improve the paraffin content of feedstreams to isomerization units because ring opening involves cleaving of only one carbon-carbon bond on the ring(s) while maintaining the same number of carbon atoms as the starting molecule. Hydrogenolysis on the other hand involves breaking one or more carbon-carbon bonds, while cracking involves cleavage of more than one carbon-carbon bond to form molecules of lower carbon number. For this application it is very desirable to use a catalyst which has high activity and selectivity to ring opening at relatively low temperatures of about 125° C. to 250° C.
Catalysts which are useful for ring opening are known and include a high chloride platinum component dispersed on a refractory inorganic oxide which is described in U.S. Pat. No. 5,463,155. U.S. Pat. No. 5,811,624 describes a catalyst for the selective opening of 5 and 6 membered rings which consists of a transition metal catalyst selected from the group consisting of carbides, nitrides, oxycarbides, oxynitrides, and oxycarbonitrides. The transition metal is selected from the group consisting of metals from Group IVA, VA, VIA of the Periodic Table of the Elements. U.S. Pat. No. 6,235,962 B1 discloses a catalyst for ring opening which comprises a carrier consisting of alumina, a metal modifier selected from the group consisting of scandium, yttrium and lanthanum, and at least one catalytically active metal selected from the group consisting of platinum, palladium, rhodium, rhenium, iridium, ruthenium, and cobalt. U.S. Pat. No. 5,382,730 discloses a process for ring opening and isomerization of hydrocarbons where the catalyst comprises an aluminosilicate zeolite such as Zeolite Y or Zeolite Beta and a hydrogenation component. U.S. Pat. No. 5,345,026 discloses a process for conversion of cyclic hydrocarbons to non-cyclic paraffin hydrocarbons where the catalyst comprises a hydrogenation-dehydrogenation component and an acidic solid component comprising a group IVB metal oxide modified with an oxyanion of a group VIB metal. U.S. Pat. No. 3,617,511 discloses a catalyst for conversion of cyclic hydrocarbons to paraffins where the catalyst comprises rhodium or ruthenium on a halogen promoted refractory oxide. U.S. Pat. No. 6,241,876 discloses a ring opening catalyst which comprises a large pore crystalline molecular sieve component with a faujasite structure and an alpha acidity of less than one and a Group VIII noble metal. US Publication No. 2002/43481 A1 discloses a catalyst for naphthene ring opening which comprises at least one Group VIII metal selected from iridium, platinum, rhodium and ruthenium on a refractory inorganic oxide substrate containing at least one of an alkali metal and alkaline earth metal. US Publication No. 2002/40175 A1 discloses a naphthene ring opening catalyst comprising a Group VIII metal selected from iridium, platinum, palladium, rhodium, ruthenium and combinations thereof. With the metal being supported on the substrate comprising at least one of a Group IB, IIB, and IVA metal. US Publication No. 2002/50466 A1 discloses a naphthenic ring opening catalyst comprising iridium in combination with at least one of platinum, rhodium and ruthenium. Finally, US Publication No. 2002/63082 A1 discloses a process where a naphtha feed is first contacted with a ring opening catalyst containing a Group VIII metal and then taking the product and contacting it with a cracking catalyst.
Applicants have developed a catalyst comprising platinum, ruthenium and a modifier such as cerium or rhenium dispersed on a refractory inorganic oxide support. Applicants have discovered that combining ruthenium with platinum results in a synergistic effect, which effect can be enhanced by addition of a modifier component. The catalyst is further characterized in that at least 50% of the platinum and ruthenium components are present as particles where the surface of the particles are enriched in ruthenium versus the center of the particles.